@keyframes change{
    0%{
        background: green;
    }
    55%{
        background: red;
    }
    100%{
        background: yellow;
    }
} 

#app4 .circle{
    border: 3px solid black;
    width: 10vw;
    height: 10vw;
    margin-left:10vh;
    margin-top:10vh ;
    border-radius: 50%;
    
}

#app4 .circle.active{
    animation: change 1s infinite alternate linear;
}